1. Field of the Invention
This invention pertains to a method and apparatus for detection of seismic and electromagnetic waves in making geophysical measurements.
2. Description of the Prior Art
It is known to use seismic and electromagnetic techniques in making geophysical measurements. Most seismic prospecting is accomplished by the generation of acoustic waves from one or more seismic sources located at or near the earth's surface. These acoustic waves are known to be reflected by interfaces or discontinuities in the subterranean formations so as to be returned to the earth's surface to be detected by one or more appropriately positioned seismic or acoustic detectors, typically geophones. Geophones are devices that detect the mechanical disturbance associated with a seismic wave. A typical geophone consists of a sensing element enclosed in an electrically insulating container. The sensing element may be of any type typically used for seismic exploration. The geophone enclosure is attached to a means for anchoring the geophone to the surface of the earth. The anchoring means may be a plate, typically constructed of metal, or, more often, a stake that may be driven into the ground. Alternatively, geophones or hydrophones may be used down a fluid-filled borehole. Hydrophones may be used offshore.
It is known that some of the reflected seismic waves are called shear waves (s-waves) and other of the reflected seismic waves are called compressional waves (p-waves). Shear waves and compressional waves differ from each other in their respective propagation speed, their angles of reflection and the acoustical vibrational directions of the particles in the layered formations through which the waves pass. The frequency range for relatively deep prospecting is low (e.g., less than about 300 Hz.) because it is well known that the higher frequencies are greatly attenuated by the formation media.
Another scheme that has been employed in petroleum exploration, using different phenomena from the detection of acoustic waves with geophones just described, is electroseismic prospecting or ESP. This technique is described in U.S. Pat. No. 4,904,942, A. H. Thompson, issued Feb. 27, 1990. The physical process required for ESP is the conversion of seismic energy to electromagnetic energy of significant value. The theory behind this technique is that there is a molecular chemical-bond attraction between the fluid and the pore surface of the solid formation, which bond is distorted or broken with the rapid movement of the fluid upon contact by an acoustical wave front, thereby inducing in a dipole manner an electromagnetic response. In applying ESP, a seismic wave is generated by conventional means. The seismic wave travels into the subsurface where it interacts with porous rock containing brine or hydrocarbons. The seismic pressure gradient causes relative motion between the rock grains and the pore fluids. This relative motion distorts dipole layers on the grain surfaces resulting in an electric field that travels back to the surface of the earth where it is detected with electric field antennas. In this context an antenna is composed of two or more electrodes imbedded in the surface of the earth.
There is an essential distinction between ESP data and seismic data. Seismic data only reveal structural information related to the elastic contrast between two different lithological regions. No information is revealed about what kind of rock is present or what fluids occupy the pore space of the regions under investigation. On the other hand, ESP only works where there is mobile, conducting water in the pore space of the formation under investigation or where there is a mixture of water and hydrocarbon. ESP, therefore, yields more information about rock and fluid types.
Another technique that has been employed with respect to the detection of certain mineral deposits utilizes a seismic source that induces a voltage in the deposit due to the piezoelectric effect. In such a case, the seismic wave distorts a piezoelectric formation, like quartz, which then is polarized and emits an electromagnetic wave. No fluids are involved. Such techniques utilize relatively high frequencies and, therefore, are limited to short penetration depths.
It is known to utilize seismic and electromagnetic waves in exploration methods, however, seismic and electromagnetic measurements for gathering geophysical data are treated as two independent tests. Separate systems are designed to optimize the detection sensitivity of each measurement. As already discussed, seismic detectors are commonly geophones used on the earth's surface or down a borehole. Electromagnetic detectors typically comprise two or more metal rods driven into the earth's surface when frequencies above one (1) Hz are to be measured. To measure lower frequencies, various types of electrochemical electrode systems are often used. These include copper sulfate "pot" electrodes. Sheets of metal such as lead may also be used to measure frequencies below one (1) Hz. These sheets are typically one (1) to one hundred (100) square feet in area, are buried one (1) to ten (10) feet in the ground and are moistened with a saline solution.
Current technology treats the seismic and electromagnetic systems as completely independent systems. This is logical because the two measurements are used for different purposes and the seismic and electromagnetic frequencies are often quite different. In magnetotellurics the electromagnetic frequencies measured are typically below one (1) Hz. In seismic tests in the petroleum industry, the frequencies range from ten (10) to several hundred Hz. The seismic measurements are used for higher resolution work while the electromagnetic studies are often of much poorer resolution and are used in a broad survey mode. Further, the seismic and electromagnetic measurements are usually made at different times. In the prior art, it is perceived that the seismic and electromagnetic systems are also incompatible with each other because it is thought that each creates excess noise in the other.
Therefore, detection of electromagnetic and seismic waves in geophysical measurements has required the deployment of two sets of detectors, one for detecting seismic energy and one for detecting electromagnetic energy. This deployment of two sets of equipment is laborious and redundant. Further, using two different measurement systems does not permit the detection of the seismic and electromagnetic signals at the same location and with the same equipment.
Thus, it is the object of the present invention to provide a method for detection of seismic and electromagnetic waves at the same location to obtain both measurements. It is another object of the invention to provide an apparatus that combines seismic and electromagnetic measurement sensors into one unit or system. The present invention results in greatly simplified applications because the transmission of the detected signals to the amplifiers and recorders can be accomplished with the same equipment for both the seismic and electromagnetic signals. The present invention thereby overcomes the limitations of the prior art and provides an improved method and apparatus that enables the detection of electromagnetic fields and seismic waves at the same location.